I've been checking G for the single keyword 'ufo' for years now, just to see how the sites rank. Way back when, there were 8M pages returned, then 9M, 10M up to nearly 11M.
Over the last 2-3 months, this fell way back, in 2 or 3 stages. At each stage, a couple of million returned pages (as shown) fell back by 1-3 million. Now its bouncing around 6-8 million. At each stage, my site ranked better, from 3rd page to second and now crawling up page 2 (positions 19 to 15).
Sheer speculation: Maybe Google is removing a lot of duplicate content, scapers etc., allowing more genuine sites to advance a bit. IF so, this is a highly positive development (copycats excepted.)